Reducing the missing wedge: High-resolution dual axis tomography of inorganic materials

doi:10.1016/j.ultramic.2006.05.010

Ultramicroscopy

Volume 106, Issues 11–12, October–November 2006, Pages 994-1000

https://doi.org/10.1016/j.ultramic.2006.05.010 Get rights and content

Abstract

Electron tomography is a powerful technique that can probe the three-dimensional (3-D) structure of materials. Recently, this technique has been successfully applied to inorganic materials using Z-contrast imaging in a scanning transmission electron microscope to image nanomaterials in 3-D with a resolution of 1 nm in all three spatial dimensions. However, an artifact intrinsic to this technique limits the amount of information obtainable from any object, namely the missing wedge. One way to circumvent this problem is to acquire data from two perpendicular tilt axes, a technique called “dual axis tomography.” This paper presents the first dual axis data at high resolution for inorganic materials, and by studying a CdTe tetrapod sample, demonstrates the increase in information obtained using this technique.

Introduction

Over recent years, nanotechnology has become a key component in the field of materials science. Technological advances are yielding ever smaller, more complex and anisotropic nanostructures that need to be characterized in all three dimensions. Unlike previous transmission electron microscopy (TEM) experiments in which the analysis of bulk single or poly-crystal thin films was often undertaken assuming uniformity parallel to the beam, many nanomaterials that are now studied have a finite size and shape in all three dimensions, and are not necessarily uniform in any direction. This new demand on materials characterization has led to the development of electron tomography for a full three-dimensional (3-D) analysis of nanomaterials.

Tomography has been used in many branches of science for nearly half a century. Although X-ray tomography was first used in medicine in what is now known as the CAT scan (computer-assisted tomography) [1], it has more recently been applied to materials science and engineering to reconstruct many microscopic 3-D structures such as metallic foams [2], and in the biological sciences to reconstruct cell structures [3]. Unfortunately, due to the wavelength of X-rays and other mechanically limiting factors, conventional X-ray tomography cannot yield a resolution better than about 1 μm. On the other end of the resolution spectrum, the atom probe field ion microscope (APFIM) has been developed over many years to produce 3-D reconstructions with single atom resolution [4]. The limitations with this technique, however, are that the sample must be conducting, withstand high field stresses, and can only produce reconstructions of relatively small volumes. The optimum technique for a nanometer-scale analysis of a wide variety of materials is electron tomography.

Electron tomography has been used very successfully in the biological sciences to study many cell structures, viruses, etc. [5], [6] using primarily bright-field (BF) TEM. Attempts have been made to apply this same technique to inorganic materials, but have often led to the reconstruction artifacts arising from the violation of the projection requirement, which states that the signal used for tomographic reconstructions must be a monotonic function of a physical property [7]. In general, the intensity of BF and dark-field (DF) images of crystalline materials, whose contrast depends almost entirely upon the diffraction condition of the crystal, does not have a monotonic relationship with the thickness of the sample [8]. One way to circumvent this problem is to use an incoherent signal for image formation, such as that used in Z-contrast imaging in the scanning transmission electron microscope (STEM). Z-contrast images are formed by collecting the high-angle scattered electrons (40–100 mrad at 200 kV) on an annular dark field detector. Detecting the scattered intensity at these high angles and integrating over a large angular range effectively averages coherent effects between neighboring atomic columns in the specimen [9], [10], [11]. Therefore, diffraction contrast in the sample is minimized, and the projection requirement is fulfilled. This imaging technique yields reliable and quantifiable 3-D reconstructions of inorganic (crystalline) materials, and as such, is the technique used for all the results shown in this paper. STEM tomography has been applied to semiconducting cadmium telluride nanostructures called tetrapods. A perfect tetrapod has four legs in tetrahedral symmetry, and as such, there is a good chance that one of the legs will be in an orientation that is perpendicular to the tilt axis. The high symmetry of the tetrapod allows the effects of the missing wedge to be clearly illustrated and to demonstrate how these effects are reduced by implementing dual axis tomography.

Section snippets

Image acquisition

All the experimental data presented here were acquired on an FEI Tecnai F20 microscope at an accelerating voltage of 200 kV, equipped with a Super-Twin objective lens and a Fischione high angle annular dark field (HAADF) detector. Electron tomography must be performed using a specialized high-tilt tomography holder, with the width of the holder determined by the size of the pole piece gap. For single axis tomography, the Fischione Advanced Tomography Holder was used, which is only 4 mm wide and

Tomographic reconstruction

Once the tilt series has been acquired, the reconstruction procedure can be started. A very important first step is the alignment of the images within the data set. This can usually be achieved with a cross correlation algorithm available in most software packages. However, in some cases, the geometry of the sample is such that a satisfactory cross correlation cannot be achieved, and the alignment must be performed manually image by image, a very laborious, but necessary, task. Once the images

Results and discussion

Two perpendicular tilt series were taken of a CdTe tetrapod sample. Each tilt series contained 69 images, the first series ranging from −70° to +70°, and the second series from −65° to +70° (acquiring two perpendicular series over exactly the same tilt range from the same volume is very difficult due to the shadowing effects discussed previously). Before the acquisition of the data sets, a suitable region of the sample was chosen, and it was rotated at low magnification to ensure that the same

Conclusions

High-resolution electron tomography has proven to be a powerful technique that has great implications for all of the nanosciences, and is now becoming widely used. With single axis tomography, a resolution of 1 nm³ is attainable, making it possible to elucidate the structure–property relationships of ever smaller nanostructures. However, the amount of 3-D information that can be obtained is often sub-optimal due to a limited tilt range, and leads to reconstruction artifacts brought about by the

Acknowledgements

The authors would like to thank Paul Alivisatos for providing samples used for this project, and Nigel Browning and E.A. Fischione Instruments, Inc. for fruitful discussions and interactions. I.A. acknowledges the Royal Society and the National Science Foundation for funding in the form of Fellowships, and J.R.T. and P.A.M acknowledge the EPSRC and the Isaac Newton Trust for funding.

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Reducing the missing wedge: High-resolution dual axis tomography of inorganic materials

Abstract

Introduction

Section snippets

Image acquisition

Tomographic reconstruction

Results and discussion

Conclusions

Acknowledgements

Progr. Mater. Sci.

J. Struct. Biol.

Trends Cell Biol.

Ultramicroscopy

Ultramicroscopy

Ultramicroscopy

Ultramicroscopy

J. Theor. Biol.

J. Struct. Biol.